Mechanochemistry-driven engineering of 0D/3D heterostructure for designing highly luminescent Cs–Pb–Br perovskites

Embedding metal-halide perovskite particles within an insulating host matrix has proven to be an effective strategy for revealing the outstanding luminescence properties of perovskites as an emerging class of light emitters. Particularly, unexpected bright green emission observed in a nominally pure zero-dimensional cesium–lead–bromide perovskite (Cs4PbBr6) has triggered intensive research in better understanding the serendipitous incorporation of emissive guest species within the Cs4PbBr6 host. However, a limited controllability over such heterostructural configurations in conventional solution-based synthesis methods has limited the degree of freedom in designing synthesis routes for accessing different structural and compositional configurations of these host–guest species. In this study, we provide means of enhancing the luminescence properties in the nominal Cs4PbBr6 powder through a guided heterostructural configuration engineering enabled by solid-state mechanochemical synthesis. Realized by an in-depth study on time-dependent evaluation of optical and structural properties during the synthesis of Cs4PbBr6, our target-designed synthesis protocol to promote the endotaxial formation of Cs4PbBr6/CsPbBr3 heterostructures provides key insights for understanding and designing kinetics-guided syntheses of highly luminescent perovskite emitters for light-emitting applications.

The manuscript by Baek et al. reported a mechanochemical method to synthesize highly emissive Cs-Pb-Br composite perovskite. By carrying out an in-depth study on time-dependent evaluation of optical and structural properties during the milling procedures, they demonstrated that the "CsPbBr3-embedded Cs4PbBr6" structural motif are critical to achieve bright green emission in the intermediate reaction stages. Moreover, the kinetics underlying the formation of such embedded heterostructures was clearly disclosed based on series of time evolution tracking of XRD, PL, absorption, HRTEM, etc. Overall, the results shown in this work are very comprehensive and analytical framework may be expanded to a wide range of perovskite related materials. Therefore, I can recommend publication of this manuscript in Nature Communications after the following issues have been fully addressed.
1. In Fig. 2b, the as-synthesized Cs-Pb-Br complex shows an optimal PLQE of ~10%. This value is low compared to the generally reported CsPbBr3/Cs4PbBr6 composites (PLQE, 40%-90%). The authors should clarify more on this relatively low PLQE.
2. For the reaction kinetics diagram proposed in this paper, it seems like the intermediate product CsPbBr3 are expected to be solely observed during the mechanochemical synthesis. However, the time dependent XRD in Fig. 3a didn't verify this. The XRD peaks for both CsPbBr3 and Cs4PbBr6 emerged at the early stage of ball milling (stage 1, 20 min), suggesting that 0D and 3D perovskites formed at the same time. In other words, there might be no such intermediate state. The authors may need to comment on this.
3. Following my last comment, the XRD patterns at 20 min and 40 min show a low angle diffraction near 11.5o, which can't be assigned to any of the materials mentioned in this work. This diffraction peak disappeared after 90 min. The authors should provide more explanations.
Dear Authors, I enjoyed reading your manuscript on the mechano-chemical synthesis of Cs-Pb-Br materials -in particular your detailed analysis of the evolution of materials formation. Indeed I find you work novel and refreshing as it provides some additional insight into the origin of green luminescence observed from what I agree are probably CsPbBr3 inclusions in a higher bandgap matrix. Your correlative XRD and PL results show that the evolution of PL and XRD signatures indeed correlate and that there are also effects of changes in the PLQY to consider showing that PL is not the best quantitative probe of the amount of CsPbBr3 formed. The manuscript is very well written and can in principle be published as-is. I would like the authors to consider the following suggestions for minor amendments or things to consider: 1) Consider to also add the PL and absorption spectra of samples before milling in Figure 2. This should of course be a fairly boring flat line but would confirm, that before milling no emissive species is present.
2) As noted by M Green and co-workers in a very comprehensive publication in 2015 (J. Phys. Chem. Lett. 2015, 6, 4774−4785), in materials with modest exciton binding energies, the excitonic absorption at the absorption onset obscures the actual bandgap. The bandgap cannot be directly determined form Tauc plots and authors should maybe consider to just refer to this as the absorption onset.

[Response]
We appreciate the reviewer's valuable time and efforts for evaluating our manuscript. In the following, we summarized the point-by-point responses and changes we have made on the basis of the reviewer's comments and suggestions. Please note that we added additional remarks on page 6 of this response letter file suggesting a modification on the manuscript title and an additional minor correction that we found during this round of revision.

[Comment 1]
1. In Fig. 2b, the as-synthesized Cs-Pb-Br complex shows an optimal PLQE of ~10%. This value is low compared to the generally reported CsPbBr3/Cs4PbBr6 composites (PLQE,. The authors should clarify more on this relatively low PLQE.

[Response]
We thank the reviewer's comments. As the reviewer commented, our highest internal PLQE value of 10.7 % from the 360-min powder sample (Fig. 2b) is low compared to generally reported values of 40-90 %. However, we note that our ligand-capped CsPbBr3/Cs4PbBr6 species exhibits a PLQE value of 66.6 % ( Fig. 4b) which is within the range of reported PLQE values as mentioned by the reviewer.
In the following, we would like to suggest the potential reasons behind the low observed PLQE values of the powder samples relative to the literature values. In short, our powder samples are (1) prepared under a synthesis approach different with those to the previous reports in terms of sample washing and synthesis methods; (2) in addition, deviations in protocols for measuring PLQE values of powder samples may also contribute to the discrepancies. Also, we would like to emphasize that the absolute values of PLQE in powder samples is not the main point of concern in this part of our study, where the relative evolution of PLQE values are more relevant for systematically investigating the synthesis mechanism. In contrast, measurements on the ligand-capped nanocrystal solutions (Fig. 4b), where the measurement is free from the above extrinsic factors, give the highest PLQE value of 66.6 %, which is well within the range of the value mentioned by the reviewer.
In response to the reviewer's comments, we revised the manuscript as following.

[Response]
We appreciate the reviewer for the comment. Regarding this comment, we would like to stress that both Path A and Path B (Fig. 3d) occur simultaneously during the whole synthesis process, as long as the reactants (CsBr and PbBr2) are present in the mixture. Thus, the reaction produces both 3D CsPbBr3 and 0D Cs4PbBr6 at early stages of the synthesis (where both of the reactants are present in abundance).
However, the fact that these two reaction pathways occurring at the same time (and sharing the same reactants) means that they are in competition against each other; hence, the relative kinetics between the two pathways is the key factor in determining which pathway dominates the reaction. Since the activation energy to form 0D Cs4PbBr6 (E0D) is likely to be higher than that of the 3D CsPbBr3 intermediate (E3D), 3D CsPbBr3 is formed at a faster rate than 0D Cs4PbBr6 (see their relative weight fractions during Stage 1 in Fig. 3c). This makes Path B (which involves 3D CsPbBr3 as the intermediate product) the kinetically more favored pathway than Path A (which proceeds directly to form 0D Cs4PbBr6 as the final product).
Therefore, we propose that most of PbBr2 is initially consumed to form 3D CsPbBr3 in the early stage (Stage 1), after which the 3D CsPbBr3 (i.e., the only Br source when PbBr2 precursor is absent) and the remaining CsBr precursor react to undergo a complete conversion to 0D Cs4PbBr6 (Stage 2).
In response to the reviewer's comment, we revised the manuscript as following.
1. We added new paragraph, "Here, both Path A and Path B occur simultaneously during the synthesis process, as long as the reactants (CsBr and PbBr2) are present in the mixture. Thus, the reaction produces both 3D CsPbBr3 and 0D Cs4PbBr6 at early stages of the synthesis. However, the fact that these two reaction pathways occur at the same time and share the same reactants means that they are in competition against each other; hence, the relative kinetics between the two pathways is the key factor in determining which pathway dominates the reaction." on page 13, line 243 of the revised manuscript.
2. We note that the term 'Intermediate Product' in Fig. 3d can be misleading. So, we changed the terminology to 'Intermediate Mixture' in Fig. 3d.

Following my last comment, the XRD patterns at 20 min and 40 min show a low angle diffraction near
11.5°, which can't be assigned to any of the materials mentioned in this work. This diffraction peak disappeared after 90 min. The authors should provide more explanations.

[Response]
We thank the reviewer for the careful scrutiny of our manuscript. We have indeed overlooked the diffraction peak near 11.5°. Figure R1 below is a zoomed-in plot near 11.5° from Fig. 3a. As shown in Fig. R1, we have confirmed that this peak is located at a precise angle of 11.74°, which originates from the (002) plane of 2D CsPb2Br5. In accordance with the reviewer's comment, this peak is observed only for 20-min and 40-min sample. This peak is not present before the milling (i.e., 0 min) and it vanishes at 90-min sample. This result is consistent with our NMR result provided in Fig. 3b where the inset shows evidence of 2D CsPb2Br5 presence at 20-min sample but not at 90-min sample.
In the original manuscript, it was erroneously mentioned that the presence of 2D CsPb2Br5 was only observable through NMR and was not confirmed by PXRD. We modified the text of the manuscript regarding this point which is detailed below. Here, it is worth mentioning (as also did from the manuscript) that Section 3.3 of the Supplementary Information file demonstrates the insignificant role of intermediate-5 produced 2D CsPb2Br5 to the green PL emission of our samples. In addition, the calculated reference peaks of 2D CsPb2Br5 was presented in Supplementary Fig. 2c. Figure R1. Powder XRD patterns of 0-min, 20-min, 40-min, and 90-min samples and reference pattern for 2D CsPb2Br5 (ref. [R5]). Diffraction peak at 11.74° marked as dashed black line corresponds to the (002) plane of 2D CsPb2Br5. Experimental data (top white panel) are magnified by 3 times and calculated data (bottom gray panel) is reduced by 5 times.
In response to the reviewer's comment, we revised our manuscript as following.
1. We added a new sentence: "Note that the small intensity peak at a low angle diffraction of 11.7° for the 20-min and 40-min sample arises from the 2D CsPb2Br5 phase, which can be further supported by the following NMR analysis. We note that this small amount of 2D CsPb2Br5 does not influence the green PL emission (see Section 3.3 of the Supplementary Information)." on page 11, line 201 in the revised manuscript.
2. We changed the following original sentence "To reveal the presence of any ternary or amorphous compound other than 3D CsPbBr3 and 0D Cs4PbBr6 that cannot be detected by PXRD, magic-anglespinning (MAS) solid-state nuclear magnetic resonance (ssNMR), a sensitive characterization tool used to determine the local environments to an atomic level, was performed." to "To reveal the presence of any ternary or amorphous compound other than 3D CsPbBr3 and 0D Cs4PbBr6 that can be corroborated with the PXRD results, magic-angle-spinning (MAS) solid-state nuclear magnetic resonance (ssNMR), a sensitive characterization tool used to determine the local environments to an atomic level, was performed." on page 11, line 205 in the revised manuscript.
3. We changed the following original sentence "Importantly, the presence of 2D CsPb2Br5, which was not confirmed by PXRD, was detected in the 20-min sample with its δiso at 235.7 ppm (see the semitransparent purple strip in the inset of Fig. 3b)." to "Moreover, the presence of 2D CsPb2Br5 was detected only in the 20-min sample (i.e., in the early stage of the synthesis) with its δiso at 235.7 ppm (see the semi-transparent purple strip in the inset of Fig. 3b) and vanishes afterwards." on page 12, line 218 in the revised manuscript.
the broad audience of Nature Communications, as well as better reflecting the main achievement made in this study.

[Remark 2] -Minor correction
In the inset of Fig. 3d, the Pb-Br configuration of PbBr2 is referred to as "Edge & Corner-sharing heptahedra". Here, the word "heptahedra" should be changed to "polyhedra". On the same note, we changed the word "heptahedra" to "polyhedra" on page 14, line 257 in the revised manuscript. This modification does not alter any scientific claims in the manuscript.
In summary, we did our best to answer the reviewer's comments, and revised our manuscript appropriately.
With these, we hope our manuscript can now be accepted for the publication in Nature Communications.

[Response]
We appreciate the reviewer's valuable time and efforts for evaluating our manuscript. In the following, we summarized the point-by-point responses and changes we have made on the basis of the reviewer's suggestions. Please note that we added additional remarks on page 11 of this response letter file suggesting a modification on the manuscript title and an additional minor correction that we found during this round of revision.

[Comment 1]
1. Consider to also add the PL and absorption spectra of samples before milling in Figure 2. This should of course be a fairly boring flat line but would confirm, that before milling no emissive species is present.

[Response]
We appreciate the reviewer's suggestion, the main concern of which is to confirm the absence of any emissive species in the 0-min sample (i.e., before any milling). As expected by the reviewer, no PL emission is observed from the 0-min sample (shown as flat baseline, Fig. R2a below). Although a small absorption onset of ~532 nm was measured from the absorption spectrum (Fig. R2b), the corresponding PLQE value of the 0-min sample is less than 0.1 % (nominal value calculated as 0.07 %), thus confirming the absence of any significant emission source in this sample (see Fig. R2c). For the sake of clarity, however, we decided to add these base-line spectra for the 0-min sample in the Supplementary Information instead of overcrowding the already densely packed main Figs. 2c and 2d. Figure R2. a, PL spectra of 0-min and 5-min sample with an excitation wavelength of 365 nm. b, Absorption spectra of 0-min and 5-min sample. c, PL spectra of the 0-min sample for PLQE measurement with an excitation wavelength of 365 nm.
In response to the reviewer's comment, we added Supplementary Fig. 3 along with the corresponding explanation in the revised Supplementary Information and added a sentence "We note that sample before milling showed a near zero PLQE value of 0.07 %, indicating that no emissive properties were present beforehand (see Supplementary Fig. 3)." on page 8, line 149 in the revised manuscript.

[Comment 2]
2. As noted by M Green and co-workers in a very comprehensive publication in 2015(J. Phys. Chem. Lett. 2015, in materials with modest exciton binding energies, the excitonic absorption at the absorption onset obscures the actual bandgap. The bandgap cannot be directly determined form Tauc plots and authors should maybe consider to just refer to this as the absorption onset. [Response]